The elements belonging the same group have similar properties. We have to pass 2, 8, 8, 18, 18, 32 and 32 elements before we come across an element with similar properties. The repetition of the elements with similar properties in the order of increasing atomic number as in the periodic table is called periodicity of properties and 2,8, 18 and 32 are called magic numbers.
Causes of periodicity of properties:
- The elements belonging to same group have have the same electronic configuration. The reoccurrence of similar valence shell electronic configurations of the elements at certain regular intervals of atomic number which becomes the cause of periodicity of properties.
Trend in a period:
- In a period, the value of effective nuclear charge increases from left to right
- The increased effective nuclear charge pulls the electron cloud of the atom/ion nearer to the nucleus and thus the size of ion goes on decreasing from left to right
As for example: Li+ ( 0.60 Angstrom)> Be2+ ( 0.31 Angstrom) > B3+( 0.20 Angstrom)
Na+ ( 0.95 Angstrom)> Mg2+ ( 0.65 Angstrom) > Al3+( 0.50 Angstrom)
K+ ( 1.33 Angstrom)> Ca2+ ( 0.99 Angstrom) > Ga3+( 0.62 Angstrom)
Trend in a group:
- In going down a group of s and p-block elements the ionic radii increase with the increase of atomic number. It is due to the progressive addition of a new shell that the atomic or ionic radii increase when we proceed from top to bottom in a group. But a cation is smaller in size than its parent atom.
Li (3) = 1s22s1
Li+ = 1s2
Na+ = 1s22s22p6
K(19) = 1s22s22p63s23p64s1
Rb(37) = 1s22s22p63s23p63d104s2 4p65s1
Cs(55) = …………..6s1
As the number of shells or principal quantum number (n) increases from 2 (for Li ) to 6 ( in case of Cs), the outer most shell electrons get farther and farther away from the nucleus and hence atomic and ionic radii increase.
- A cation is formed by the removal of one or more electrons from the atom.
- Cation has a lesser number of electrons than its parent atom ( Compare Li and Li+ or Na and Na+ )
- As a result, the magnitude of screening constant decreases but the magnitude of effective nuclear charge increases.
- The increased effective nuclear charge pulls the electron cloud of the cation nearer to the nucleus and thus makes cation smaller in size than its parent atom.
- So, we can say Na+ is smaller in size than the Na atom
From the above discussion, we can be able to explain the following:
- Li+ is smaller than the Li- atom
- Cation is smaller than its parent atom